https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106542
Bug ID: 106542
Summary: -O2 sign-extended uint32 to uint64
Product: gcc
Version: 9.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: wuz73 at hotmail dot com
Target Milestone: ---
#include <iostream>
unsigned Msb(uint64_t x) { return 63 - __builtin_clzl(x); }
template<typename T>
void f(T r, T p)
{
if (p > r)
{
auto d = p - r;
unsigned k = Msb(sizeof(T) == 4 ? unsigned(d) : d);
std::cout << sizeof(T) << " k=" << k << "\n";
}
}
int main()
{
int data[] = {-1610499096,2086724600};
f(data[0],data[1]);
}
When compiled with "g++ -O2" the output is "4 k=63". Apparently before calling
Msb(), g++ sign-extended unsigned(d). static_cast<unsigend>(d) yields the same
result. However, "g++ -O0" is correct (k=31), so are earlier versions of g++
such as 4.8.5 and 5.3.1.
